As of October 2026, the Massachusetts Clean Energy Center's ADU guide estimates about $250,000 to $450,000 for a two-bedroom detached ADU, $235,000 to $360,000 for a two-bedroom attached ADU and $120,000 to $270,000 for an interior conversion, including site work. Soft costs often run 10 to 20 percent of the total. Your lot decides where you land.
What do published Massachusetts figures say?
We did not find a source that tracks Boston ADU prices alone. These are the most useful published figures, as of October 2026:
- Studio detached ADU: about $200,000 to $300,000 (MassCEC).
- Two-bedroom detached ADU: about $250,000 to $450,000 (MassCEC).
- Two-bedroom attached ADU: about $235,000 to $360,000 (MassCEC).
- Interior conversion: about $120,000 to $270,000 (MassCEC).
- One South Shore builder: $350,000 to $450,000 for its attached and detached units, or roughly $500 to $528 per sq ft (Banker & Tradesman, August 2026).
MassCEC also gives a rough planning figure of $250 to $500 per sq ft including design and fees, and estimates a kitchen at $25,000 to $50,000 and each bathroom at $15,000 to $25,000.
What costs sit on top of construction?
The builder quoted by Banker & Tradesman listed these pre-construction costs in August 2026:
- Site survey: $7,000 to $10,000.
- Building permit fees: $750 to $5,000, depending on the municipality.
- Home energy rating modeling: $1,200 to $1,500.
- ADU plans: $1,500 to $5,000.
- Water connection: $3,000 to $7,000, plus a similar amount for labor; sewer about the same.
Is there money to help pay for an ADU?
- Mass Save offers $15,000 to $25,000 in incentives for all-electric ADUs, per MassCEC. Units using gas, oil or propane are not eligible.
- MassHousing runs an ADU loan program for income-eligible borrowers.
- Boston Home Center offers ADU financial assistance for Boston homeowners.
Why might my number differ from these ranges?
It depends on what the site hides. An electrical service upgrade, a long sewer run, ledge in the yard or a basement that needs its floor lowered can each move the price a lot. That is why we visit, measure and open what we can before giving you a written, itemized quote.
Related
Sources, as of October 2026: Massachusetts Clean Energy Center, ADU Budgeting and Finance guide; Banker & Tradesman, "What Does It Cost to Build an ADU in Massachusetts?" (August 2, 2026). Figures are published estimates, not our quotes.
Updated October 2026